This term is applied when a part is used to represent a whole, or vice versa. The main subject is substituted for something inherently connected to it. This substitution only works however, if what the synecdoche represents is universally recognised and understood. An example of this would be the Statue of Liberty, which is representative of New York.
A visual metaphor is used to transfer the meaning from one image to another. Although the images may have no close relationship, a metaphor conveys an impression about something relatively unfamiliar by drawing comparison between it and something familiar. 'The big apple' is a metaphor linked to New York.
A metonym is a symbolic image that is used to make reference to something with a more literal meaning. By way of association the viewer makes a connection between the image and intended subject. Unlike a visual synecdoche, the two images bear a close relationship, but are not intrinsically linked. And unlike visual metaphors, metonyms do not transfer the characteristics of one image to the other. Yellow cabs are associated with New York, despite there being so many other places where these can be found.
Here is the imagery created for a flier for a British film festival. The illustrator that produced it has taken certain major landmarks from London, in order to convey the location easily without having to draw London...
Here is an example of a typeface designed to represent the London underground, using the design of the underground map as a basis for the face. The style of the London underground map is one that I think could be easily recognised by anywhere regardless of their language so I think it is a good way to symbolize London's underground. The underground itself could also be seen as a metonym for London as a city because although lots of cities have a metro or underground system, none are quite as well known as London's.